1. Field of the Invention
The present invention relates to a solid-state imaging device, a method of manufacturing a solid-state imaging device, and an electronic apparatus. In particular, the invention relates to a solid-state imaging device, a method of manufacturing the solid-state imaging device, and an electronic apparatus using the solid-state imaging device by which images with high quality can be obtained.
2. Description of the Related Art
Solid-state imaging devices have been incorporated in various electronic apparatuses, for example, imaging apparatuses such as digital still cameras, video cameras, etc. and mobile terminal apparatuses having an imaging function, for a variety of uses. The solid-state imaging devices include APS (active pixel sensor) having amplifying elements on a pixel basis, and CMOS (complementary MOS) image sensors in which signal charges accumulated in photodiodes provided as photoelectric conversion elements are read out through MOS (metal oxide semiconductor) transistors are in wide use.
In a CMOS image sensor, in general, a readout operation of reading out the signal charge accumulated in each photodiode is carried out on the basis of each row in a pixel array, and the pixels for which the readout operation is finished restart accumulation of electric charge at the moment the readout operation is finished. Since the readout operation is thus performed on the basis of each line in the pixel array, in the CMOS image sensor, the signal charge accumulation periods cannot be made coincident for all the pixels. In the case where the subject to be imaged is moving or in other similar situations, therefore, the image obtained would be distorted. For instance, when a subject which is straight shaped in the vertical direction and which is moving horizontally is shot, the subject is imaged as if it were inclined.
In order to obviate the generation of such a distortion of image, there has been developed an all-pixel-simultaneous electronic shutter for CMOS image sensor such as to realize the same exposure period for all the pixels. The all-pixel-simultaneous electronic shutter is a shutter ensuring that exposure is started simultaneously and finished simultaneously for all the pixels effective for imaging, and such a system is also called global shutter (global exposure). Systems for realizing the global exposure include mechanical systems and electrical systems.
For example, in a mechanical system for global exposure, a mechanical shutter (light blocking section) capable of opening and closing for blocking light on the front side of the CMOS image sensor is utilized. Specifically, the mechanical shutter is opened, to start exposure simultaneously for all the pixels. At the end of the exposure period, the mechanical shutter is closed to shield all the pixels from light simultaneously. Consequently, the period of generation of light electric charge in the photodiode is coincident for all the pixels.
On the other hand, in an electrical system for global exposure, an electric-charge discharging operation for emptying the accumulated charge out of the photodiode is carried out simultaneously for all the pixels, and exposure is started. At the end of the exposure period, driving of a transfer gate is conducted simultaneously for all the pixels so that the accumulated light electric charges are wholly transferred into floating diffusion layers (capacitances), and the transfer gates are closed. As a result, the period of generation of light electric charge in the photodiode is coincident for all the pixels.
In the electrical system, however, the transfer of the electric charge into the floating diffusion layer which is carried out simultaneously for all the pixels is attended by difficulty in removing noises, which may lead to deterioration of image quality. In order to restrain such deterioration of image quality, a pixel structure having a memory part has been proposed.
In the pixel structure having a memory part, an electric-charge discharging operation for emptying the accumulated charge from the photodiode is carried out simultaneously for all the pixels, and exposure is started. At the end of the exposure period, driving of the transfer gate is conducted simultaneously for all the pixels so that the accumulated light electric charges are wholly transferred into the memory parts, to be held there. Then, after the floating diffusion layers are reset, the electric charges held in the memory parts are transferred into the floating diffusion layers, and readout of signal levels is conducted.
By the pixel structure in which the memory part for temporarily holding the electric charge accumulated in the photodiode is thus provided separately from the floating diffusion layer, it is possible to reduce noise. However, the addition of the memory part to the inside of the pixel leads necessarily to a reduction in the area of the photodiode, resulting in a decrease in the maximum amount of electric charge (maximum charge amount) that can be accumulated in the photodiode.
In view of the foregoing, the present applicant has proposed a pixel structure in which a photodiode and a memory part are integrally united together through an overflow path, in order to obviate a reduction in the maximum charge amount of a photodiode (see, for example, Japanese Patent Laid-open No. 2009-268083, referred to as Patent Document 1 hereinafter).
Meanwhile, in the solid-state imaging device proposed in Patent Document 1, a structure is adopted in which a depletion state is obtained while forming a potential barrier in a charge transfer path between a photodiode and a memory part. Specifically, the structure has an impurity diffusion layer forming a potential barrier such that overflow of the light electric charge generated in the photodiode to the memory part occurs when an electric charge in excess of a predetermined charge amount is accumulated in the photodiode, even in the condition where a voltage sufficient for turning-OFF of a first transfer gate present between the photodiode and the memory part is impressed.
Such a structure makes it possible to suppress the deterioration of image quality due to noise, and to obviate a reduction in the maximum charge amount in a photodiode.